Not This Month, Honey: Why You Shouldn’t Conceive a Baby in May

It turns out that it does matter what time of year you’re born — at least when it comes to your health.

We’re not talking about in the Athena Starwoman horoscope way, but in actual measurable health data. And what’s important may not be the month you’re born in but more the month in which you were conceived (which, after all, does dictate when you’re born).

According to the study from Princeton University, there’s a 10% higher rate of prematurity among babies conceived in May. The authors, Janet Currie and Hannes Schwandt, conclude that this may be because the moms hit their third trimester around flu season, and flu is a known factor in early delivery. Prematurity can contribute to a higher risk of asthma, learning disabilities and other developmental problems later in life. The two researchers also found that babies conceived in summer months were almost a third of an ounce (8 g) heavier, which is a tiny amount, but when you only weigh a few pounds, every fraction of an ounce counts.

Currie and Schwandt, researchers at Center for Health and Wellbeing at Princeton University, who reported their findings in the Proceedings of the National Academy of Sciences, compared more than 1.4 million siblings born to 647,050 mothers in New York City, New Jersey and Pennsylvania. The seasonal difference in birth outcomes has been found in previous studies, but those analyses pointed to factors like the socioeconomic status of the mother since wealthy, educated and nonteenage mothers usually have access to better prenatal care. But because the scientists compared brothers and sisters with similar genetic and socioeconomic environments, these factors were somewhat neutralized. “A high[-socioeconomic-status] mother getting pregnant in an unfavorable month will, on average, experience similarly poor birth outcomes as the typical (lower socioeconomic status) mother conceiving in [an] unfavorable month,” the study authors write.

Why does the flu make mothers deliver early? It’s possible that inflammation, which increases in response to the flu and has been linked in other studies with early delivery (and in one recent study, with autism), may be behind the premature birth. But while the flu may explain the premature births for babies conceived in May, it’s less clear what causes the summer-conception weight gain. “Women gain almost 1 lb. more when they conceive in June, July, or August than when they conceive in January,” says the study, “suggesting that gains in birth weight are driven, in part, by higher maternal weight gain during pregnancy.” The authors surmise this has to do with seasonally available nutrition (more ice cream during the hot summer months, perhaps?). They also found that smoking or marital status didn’t seem to be contributing to their results.

More studies are documenting the potentially important role that birth month may play in health; one recent analysis found a link between birth month and immune disorders, but this study provides some added weight to the idea, since it studied kids from the same moms.

So is timing really everything? Perhaps. At the very least, it’s one of the many things worth considering if you’re thinking of starting a family. And no matter what month you conceive, it’s important to get that flu shot.

if the exposure to the flu virus can cause all of this prematurity, autism and poor outcomes, why in the world would a flu shot exposure be any different? The virus will be circulating in the mom's bloodstream, wreaking all the havoc on her immune system and having the same outcome as getting the flu itself on the developing baby. Why get a flu shot? Flu virus is also linked to schizophrenia. Our healthcare system is seriously wacked!

A kind of traumatic side-effect of my sex-ed. course in high school was learning how long gestation takes. I counted back from my birthday and realized exactly what it was my mother gave my father for his birthday.

Meh. Trying to manipulate the baby's month of birth seems as crazy/compulsive as trying to manipulate the baby's gender. Young mother worry about too much stuff. Get in a stable marriage, get healthy, get your vaccinations, get pregnant. Sure, do what you can with the big stuff, but manipulating every little probability factor seems compulsive, as if the monster bride and helicopter mom think are carried to even greater extremes. Relax, be good, and enjoy life.

Concluding Sentence: "So is timing really everything? PERHAPS [capslock added]. At the very least, it’s one of the many things worth considering if you’re thinking of starting a family."

In other words, the author makes an assertion in her title, and then blatantly walks back that assertion in her concluding sentence. She sounds more ambivalent than sure of herself.

Just a word to the wise at TIME Magazine: If you continue to post these lovely pieces of drivel, you will continue to bleed revenues and readership, because (quite frankly) a high school student could write a better article than this one.

@ShellyS Well flu shot is dead flu virus of that year strand.. They evolve every year you see. So by getting a flu jab it is in theory prepare your white blood cells to kill off the year strand of flu virus efficiently so you dont actually get sick.. But some people have really weak immune system that they still get sick from the flu jab but not as bad as catching an active flu virus..

@BorisIII Basic principles of biology (such as taught in the 10th grade in my state) dictate that no one gets a gene that way. Once you are conceived, those are the genes you have. Mom getting the flu weeks or months later can not change the fetal genes.